Digitalis preparations such as digoxin and digioxin have been conventionally used as cardiotonics in treating cardiac insufficiency (eee, e.g., Iyakuhin Yoran, pp. 324-327, 1977, Yakugyo Jiho Sha). While other cardiotonic compounds such as nicotinonitrile derivatives (see, e.g., Japanese patent Laid-Open No. 70868/1982), imidazolone derivatives (see, e.g., Japanese patent Laid-Open No. 155368/1984and dihydropyridazinone derivatives (see, e.g., Japanese patent Laid-Open No. 74679/1983) have been reported.
Digitalis preparations presently used in the above treatment have a very limited range of safety so that they should be handled by a skillful person. In addition, it has been reported that these preparations might exhibit some side effects including arrhythmia. On the other hand, the nicotinonitrile derivatives, imidazolone derivatives and dihydropyridazinone derivatives as recently reported have some disadvantages such as a low cardiotonic effect, a very limited range of safety, an effect of increasing myocardial rhythm or a high toxicity in animals.